Calculate A+B-C+D

Sally was driving home from one of her business trips in Northern Arizona when she saw an elderly Navajo woman walking on the side of the road. As the trip was a long and quiet one, she stopped the car and asked the Navajo woman if she would like a ride. With a silent nod of thanks, the woman got into the car.

Resuming the journey, Sally tried in vain to make a bit of small talk with the Navajo woman. The old woman just sat silently, looking intently at everything she saw, studying every little detail, until she noticed a brown bag on the seat next to Sally.

'What in bag?' asked the old woman. Sally looked down at the brown bag and said, 'It's a bottle of wine. I got it for my husband.'

The Navajo woman was silent for another moment or two. Then speaking with the quiet wisdom of an elder, she said, 'Good trade.'